This invention relates to a cement composition which can be directly placed under water and to such a composition which can be pumped.
This invention relates to an improvement in a cement composition which can be directly placed under water or in a watery place. Particularly, this invention relates to an underwater concreting cement composition having improved flow during placing and an improved hardening rate. Moreover, this invention provides an underwater concreting cement composition which does not corrode reinforcements or steel frames.
Up to this time, when ready mixed concrete was directly placed under water without using any tremie or enclosure, the cement was washed out during the placing by the surrounding water and was dispersed in the water, or it was dispersed or washed out after the placing by a water stream or waves before it hardened. A deformation of the concrete or a partial loss of the cement portion from the concrete occurred whereby to effect unsatisfactory hardening, so that it was only possible to obtain concrete having markedly lowered strength after hardening.
In recent years, a concrete has been proposed which can be directly placed in a desired place by shooting it through water by using an additive which imparts viscosity to cement.
For example, the specification of West German Patent Publication No. 2,326,647 describes that the dispersion of cement into water can be prevented by adding thereto a cellulose ether, polyacrylamide or the like and discloses a formulation containing hydroxyethylcellulose as an example. Further, Japanese Patent Laid-Open No. 3921/1982 relates to a process for placing an underwater concrete and shows a concrete formulation containing polyacrylamide as an example.
On the other hand, in many of the sites in Japan where underwater concreting must be applied, it is generally difficult to provide a working shop and a staging which are convenient for preparing a concrete composition and for shooting it directly to the area in which it is to be placed and, therefore, it is an essential requirement to use ready mixed concrete that can be pumped.
An important factor for preventing the dispersion of cement into water is to increase the viscosity of the cement composition, and this purpose can be attained by the addition of a water-soluble polymer.
When polyacrylamide is added to cement as in the specification of Japanese Patent Laid-Open No. 3921/1982, the dispersion of the cement into water can be reduced effectively but the flow of the cement is markedly worsened so that it is difficult to obtain a pumpable composition.
A number of cellulose ethers constitute examples of the polymer additives which are known as cement additives. Namely, the addition of a cellulose ether is effective in increasing the viscosity as well as flow, so that it is used to improve the application workability of mortar (see, for example, Japanese Patent Laid-Open Nos. 130823/1975 and 22666/1981). Further, a variety of cellulose ethers, such as ethylcellulose, methylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, methylhydroxyethylcellulose, or methylhydroxypropylcellulose can be added to cement. However, when these cellulose ethers are used as cement additives, an air-entraining property is imparted to the cement and, therefore, when a sufficient quantity of the ethers is added to prevent dispersion of the cement into water, the quantity of air entrained by the mortar or concrete becomes too high, resulting in lowering of the strength of a structure after hardening. In order to lower the quantity of air entrained by this cement/cellulose ether composition, the addition of a defoamer is effective, but the flow of the composition is lowered as well, because the flow of the cement/cellulose ether composition is also dependent upon the quantity of air. It has therefore been thought to be very difficult to obtain a mortar or concrete composition which is resistant to dispersion into water, has good flow properties and a low entrained air quantity.
Further, West German Patent Publication No. 2,326,647 describes the addition of both a polymer additive and a cement setting accelerator to an underwater concreting cement composition and discloses calcium chloride, sodium aluminate or alumina cement as the accelerator. However, calcium chloride, though inexpensive, has a drawback that it dissolves in water to form chloride ions which corrode iron and so raises a great practical restriction in that it can not be used in structures under water or in watery places, such as reinforced concrete or concrete in contact with iron. Moreover, the use of the polymer additive together with sodium aluminate or alumina cement, which is an accelerator noncorrosive to iron, has a drawback that the flow of an underwater concreting cement composition is markedly lowered. It has, therefore, been thought to be very difficult to obtain an underwater concreting cement composition which does not cause the dispersion of cement into water, has good flow, does not corrode iron and has a suitable setting time.
As a result of an intense study, the inventors of this invention have recognized that a composition having a good flow, a good resistance to dispersion into water and a low air-entraining property, that is, a composition suited as underwater concreting cement, can be obtained by adding cellulose ethers, especially hydroxyethylcellulose, together with a high-molecular weight polyethylene oxide to cement and thereby have reached this invention. Further the inventors have found that, surprisingly, the flow of a cement composition can be improved and at the same time the setting time can be shortened by combining hydroxyethylcellulose, which is the polymer additive, with calcium formate which is a cement setting accelerator noncorrosive to iron, and have accomplished this invention.